1. Field of the Invention
This invention relates to a deceleration limiter, particularly for a turbine engine.
2. Description of the Prior Art
It is known that the fuel metering valve of a turbine engine is normally controlled using a throttle lever which acts on a fuel controller, various conventional embodiments of which exist. This fuel controller may be, for instance, a speed sensor balance, the beam balance of which is affected by various control elements which act on both of its arms. Most of these control elements receive input data originating from sensors which detect various operating parameters of the turbine engine. The throttle lever acts either directly or indirectly on the speed sensor balance beam through a compressed elastic element, such as a spring.
It is also known that the operating conditions of a turbine engine are governed by the fuel mixture richness, which varies in the same way as the ratio (C/P) of the fuel flow (C) emitted by the injector at a specific pressure (P), to the flow of air traversing the combustion chamber, specifically at the compression pressure (P2) of the compressor. In particular, the specific ratio (C/P) must not fall below a set minimum, to avoid risks of extinction of combustion in the combustion chamber due to a lean mixture.
For this reason, the fuel controller which acts on the fuel metering valve is normally equipped with a special device, designated by the name of "deceleration stop," the function which is illustrated in FIG. 1. This diagram shows the variations of the specific ratio (C/P) as a function of the rotation speed (N) of the turbine under various operating conditions. Curve S corresponds to steady operation of the turbine engine, for instance, at a fixed load and at uniform cruising speed in the case of a turbojet installed in an airplane. Curve Li corresponds to the lower limits imposed on the specific ratio (C/P) by the deceleration stop device, whereas curve Ls corresponds to the upper limits imposed on the C/P ratio by an acceleration stop device.
FIG. 2 shows, in the same plane of coordinates as FIG. 1, a solid curve (P) corresponding to abrupt deceleration which, when the pilot abruptly activates the throttle lever in the corresponding direction, causes the operating point of the turbine engine to move from point A to point B of its steady operation curve (S). Curve P in the FIG. 2 diagram clearly illustrates that although fuel controllers used prior to the present do perform the so-called "deceleration stop" function corresponding to the Li curve, they do not prevent a very sudden and very rapid decrease of the specific ratio (C/P), and, subsequently of the flow (C) of fuel emitted by the metering valve to the turbine engine. It is apparent, and it has in fact been noted, that decreases of such speed and magnitude of the fuel flow subject the turbine engine to extremely abrupt changes in operating conditions which translate particularly into thermal shocks which sometimes result in damage to the turbine engine and, in any case, substantially reduce the longevity of its various parts.
U.S. Pat. No. 3,596,466 to Anschutz et al. discloses a fuel controller for a turbine engine fuel fuel metering valve, wherein the fuel metering valve itself is controlled by a valve, which in turn receives, from a mechanical device, a signal proportional to the square root of the specific ratio (C/P). This prior art device is therefore complex which, in the technology under consideration, causes inconveniences in terms of reliability and maintenance.
U.S. Pat. No. 3,513,899 Paduch also discloses a fuel controller which acts on a turbine engine fuel metering valve through a step-up gear. In case of maximum deceleration, the action of the throttle lever on this gear is limited by an extremely complex gearing system, which comes to rest against a three-dimensional cam which defines the rate of variation of the specific ratio (C/P) during deceleration. This three-dimensional cam moves axially as a function of the turbine engine compressor speed, while its angle is affected by the temperature at the compressor inlet. Again, the complexity of this device is unsatisfactory in terms of reliability and maintenance.